Positron emission tomography, also known as a PET scan, is a diagnostic examination that involves the acquisition of physiologic images based on the detection of radiation from the emission of positrons. PET scans produce digital pictures that can, identify many forms of cancer, damaged heart tissue, and brain disorders such as Alzheimer's, Parkinson's, and epilepsy. Technically, PET is a imaging technology that images the biology of disorders at the molecular level before anatomical changes are visible. A PET scan can distinguish between benign and malignant disorders (or between alive and dead tissue), unlike other imaging technologies which merely confirm the presence of a mass. PET scans give information about the body's chemistry not available with other imaging techniques. PET scans reveal metabolic information as opposed to anatomical information.
A short-lived radioactive tracer isotope which decays by emitting a positron, chemically combined with a metabolically active molecule, is injected into the patient (usually into blood circulation). There is a waiting period while the metabolically active molecule (usually a sugar) becomes concentrated in tissues of interest, then the subject is placed in the imaging scanner. The short-lived isotope decays, emitting a positron. After travelling up to a few millimeters the positron annihilates with an electron, producing a pair of annihilation photons (similar to gamma rays) moving in opposite directions. These are detected when they reach a scintillator material in the scanning device, creating a burst of light which is detected by photomultiplier tubes. The technique depends on simultaneous or coincident detection of the pair of photons: photons which do not arrive in pairs (i.e., within a few nanoseconds) are ignored. By measuring where the annihilation photons end up, their origin in the body can be plotted, allowing the chemical uptake or activity of certain parts of the body to be determined. The scanner uses the pair-detection events to map the density of the isotope in the body, in the form of slice images separated by about 5mm. The resulting map shows the tissues in which the molecular probe has become concentrated, and is read by a nuclear medicine physician or radiologist, to interpret the result in terms of the patient's diagnosis and treatment. PET scans are increasingly read alongside CT scans or MRI scans, the combination giving both anatomic and metabolic information (what the structure is, and what it is doing). PET is used heavily in clinical oncology (medical imaging of tumours and the search for metastases).
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PET can detect disease sooner. Prior to changes in structure that normally would show up on a CT or MRI scan, a PET scan can reveal metabolic changes in the body. Cancer is a metabolic process and PET is a metabolic imaging technique.
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PET shows the extent of disease - called staging - of lung cancer, colorectal cancer, melanoma, head and neck cancer, breast cancer, lymphoma and many other cancers. For patients whose cancer is newly diagnosed, it is important to determine if the cancer has spread to other parts of the body so that appropriate treatment can be started. PET can search the entire body for cancer in a single examination with a "whole body scan," revealing the primary site(s) as well as any metastases.
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PET shows whether a tumor is benign or malignant. Reports in scientific literature find that, in some tumors, PET correctly identifies detected lesions 95% of the time. Painful, costly and invasive surgery, such as thoracotomy, may no longer be necessary for diagnosis.
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PET shows the effectiveness of therapy. It is an excellent way to monitor progress and test recurrence of disease.
For more information on the PET/CT Scanner at Sebastian River Medical Center, call 772.589.3186, ext. 5440.
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